西藏岗巴地区海相上白垩统碳、氧同位素对比实验结果偏差分析

Deviation Analysis of the Comparable Carbon and Oxygen-Isotopic Values of the Upper Cretaceous Limestones from Gamba, Tibet, China

对西藏南部岗巴地区上白垩统部分全岩样品进行了碳、氧同位素不同实验室、不同时间的对比实验,目的在于了解样品处理过程可能对试验结果的影响程度。结果发现,不同实验室的结果偏差比同一实验室不同时间的结果偏差大,碳同位素比值的偏差比氧同位素比值的偏差大。不同实验室结果偏差的关键原因是样品制备方法和岩性的不同:手工碎样、选样、磨样较难避免微裂隙中充填的方解石脉和小型生物扰动组构对同位素值的影响,而毫米级直径钻头刮样取样则很大程度上可以避免;钙质页岩和泥灰岩受影响的程度相对较小,生物碎屑灰岩受影响较大。碳同位素比值偏差大于氧同位素,前者基数比后者大4-5倍可能是主要原因。

Three sets of carbon and oxygen isotopic values for ten samples, which are the Upper Cretaceous sedimentary rocks taken from the Zongshan section in the Gamba area, Tibet, China,obtained respectively at both the Tongji University of China and the Oxford University of UK. A comparative analysis is to check how much influence has been caused to the carbon and oxygen isotopic values of various samples by different powder sample processing. Powder samples, for analyzing carbon and oxygen isotopic values, were prepared respectively through hand breaking, selecting and abrading procedures at the Tongji University both in 2003 and 2004, whereas powder samples were prepared through scratching by a small drilling machine at the Oxford University in 2004. A comparison among the carbon and oxygen isotopic values, which were measured in various laboratories or in different years at the same laboratory, indicates that the deviations of the data obtained in two laboratories respectively are bigger than those of the data obtained respectively at the same lab in different year, with the deviations of the carbon isotope values even bigger than those of the oxygen isotopic value of the relevant samples. It is believed that these deviations were caused by the differences of the sample preparation methods and lithologies of samples. This is because that the sample powders prepared through hand breaking, selecting and abrading processes could contain secondary calcite which is filled as veinlets in microcracks and bioturbation fabrics, whereas the powders prepared through scratching by a small drilling machine could avoid to contain secondary calcite. The deviations of the carbon and oxygen isotopic values of calcareous shale and marlstone are small, whereas those of the bioclastic limestone are quite big. It is suggested that the much bigger deviation percentages of the oxygen isotopic values in comparison with those of the carbon isotopic values could be caused by the deviations of the carbon isotopic values 4-5 times higher than those of the oxygen isotopic values.